Connection or supply relationships among different devices in a mechanical, electrical, and plumbing (MEP) system can provide important information for various applications. These applications include Auto Fault Detection (AFD), Energy Consumption Monitoring, Prediction and Simulation, and Graphics of a Duct Network. While it is feasible to get the connection relationships via the IfcRelationship and 3D Geometry analysis if building information model (BIM) data are available, most legacy buildings don't have BIM data. Rather, these buildings only have CAD drawings (which are a raster type image (such as BMP, JPG or TIF) or a vector-based file (such as DXF or SVG)) or even just paper drawings (which can be scanned into a raster image).
It is costly to hire architectures or domain experts to manually separate the devices from the paper-based drawing with commercial tools like Revit, and then analyze and determine the connection relationships. Moreover, with a manual process, it is difficult or infeasible to update the model when the CAD drawing is revised.